1. Field of the Invention
This invention relates in general to digital signal transmission, and more particularly to an apparatus for recovering timing of a digital signal for a transceiver.
2. Description of Related Art
The telephone networks currently in place were originally designed for transmission of electrical signals carrying human speech. Since human speech is generally confined within a band ranging from 0 Hertz to 3,400 Hertz, telephone networks were designed to provide telephone lines to each user which were capable of handling frequencies within this range. Today, these same telephone lines, which connect a service user to a central office, are in place, permitting communication of only voice data or analog modem transmissions of not more than 56,000 bits per second. However, connections between central offices of telephone networks are provided by high-bandwidth fiber optic transmission facilities in nearly every telephone network worldwide.
Because the equipment that connects an end user to a central office is only capable of handling frequencies of up to 3,400 Hertz, communication equipment utilizing these lines, such as dial modems or fax modems, have been accordingly limited in bandwidth. Despite the presence of high bandwidth fiber optic lines between central offices, users remain limited in the bandwidth available to them because the local lines serve as a bottleneck. New technologies, such as the Internet access or video conferencing, demand that the bottleneck be removed.
Digital Subscriber Line (DSL) technologies are capable of removing the bottleneck. DSL permits a user to communicate over the existing telephone lines at a rate of tens of millions of bits per second. In order to utilize DSL, a site must be equipped with a transceiver (a DSL modem) which communicates, via the existing telephone lines, with another transceiver located at the central office of the network access provider, generally the local telephone company.
As already stated, DSL transceivers are designed to operate at high data rates. The DSL transceiver operates either in a network (central office) or remote mode. In a network mode, a transceiver only needs to acquire the phase of the received signal because the received signal is frequency locked at the remote transceiver to the network transmitter. Further, in the remote mode, the transceiver must frequency and phase lock to the incoming data stream. The transceiver may also be capable of variable rate transmission.
To simplify the transceiver structure and to allow a variable rate of transmission, it can be seen that there is a need for an apparatus for performing timing recovery wherein the baud rate conversion is independent of the sampling rate.
It can also be seen that there is a need for recovering timing of a digital signal for a transceiver wherein the timing recovery system may be set in a network mode or a remote mode.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an apparatus for recovering timing of a digital signal for a transceiver.
The present invention solves the above-described problems by providing an apparatus for performing timing recovery wherein the baud rate conversion is independent of the sampling rate. The timing for a digital signal at a transceiver is determined by the timing recovery system, which may be set in a network mode or a remote mode.
A system in accordance with the principles of the present invention includes a transceiver core having an input side and an output side, the transceiver core processing transmit and receive data at a predetermined baud rate, an analog front end, coupled to the transceiver core, for transmitting and receiving analog signals over a network, a phase detector, coupled to the transceiver core, the phase detector generating a phase error estimate signal based upon a sampling of a received signal at the transceiver core and a timing controller, coupled to the phase detector, for receiving the phase error estimate signal and generating a receive and transmit phase control signal for controlling timing of the analog front end, wherein the timing or sampling of the analog front end is independent of the baud rate. The timing controller may further include a selector for selecting a remote mode of operation or a network mode of operation.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the analog front end further includes a transmit converter for converting the transmit data at the baud rate to a digital output at a transmit rate and a digital to analog converter, coupled to the transmit converter, for converting the digital output to an analog signal.
Another aspect of the present invention is that the analog front end further includes an analog to digital converter for converting the analog receive signal to a digital receive signal and a receive converter for converting the digital receive signal at a receive rate to the baud rate.
Another aspect of the present invention is that the digital to analog converter is clocked at a predetermined frequency according to a clock input signal.
Another aspect of the present invention is that the phase detector further includes a channel estimator for generating a coefficient error signal that is used to tune the channel estimator and for generating a phase reference value, and a comparator for receiving the phase reference signal from the channel estimator and a nominal reference value for processing to generate a phase error estimate of the received signal.
Another aspect of the present invention is that the channel estimator includes a linear equalizer having a plurality of taps, the taps being weighted according to the coefficient error signal, the weighted taps being summed by a first summer to generate a composite channel estimate signal and an error generator, coupled to the linear equalizer, for receiving the composite channel estimate and calculating the difference between the composite channel estimate and a channel reference value to produce the coefficient error signal.
Another aspect of the present invention is that the timing controller further includes a loop filter for filtering the phase error estimate and producing a control signal, a numerically controlled oscillator for producing a baud interrupt signal and the phase control signals, and a selector, wherein the selector further includes a network input signal and a remote input signal, the network input signal being applied to select the network mode and to lock to the phase of the receive signal by running the numerical controlled oscillator at a nominal numerical controlled oscillator frequency with a variable phase offset, and the remote input signal being applied to select the remote mode and to lock to the frequency and phase of the receive signal by setting the frequency of the numerical controlled oscillator using the output of the loop filter.
Another aspect of the present invention is that the analog front end further includes a transmit converter, the transmit converter converting the timing of the transmit data from the transmit baud rate to a predetermined transmit signal frequency by interpolating the transmit baud rate signal by a fixed amount, filtering the interpolated signal to remove alias noise and decimating the filtered signal by a variable amount.
Another aspect of the present invention is that the analog front end further includes a receive converter, the receive converter converting the timing of the receive signal to the receive baud rate by interpolating the receive signal by a fixed amount, filtering the interpolated receive signal to remove alias noise, and decimating the filtered signal by a variable amount.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.